Radio frequency (RF) receivers receive a broad spectrum of input signals from the receiver antenna. In the process of selecting the signals of interest from the input spectrum, the incoming RF signals are typically filtered through a bandpass filter to narrow the frequency of input signals to be processed by the receiver. A automatic gain control (AGC) is used in conjunction with a low noise amplifier (LNA) on the front end of the RF receiver to select the appropriate gain for the LNA to apply to condition or equalize the incoming signal gain and assist in placing the incoming signal into the appropriate linear region for signal conversion by the receiver. Because the input signal strength can vary greatly over the input frequency, AGC is an adaptable system that uses feedback signals to locate the appropriate gain level for the LNA to apply.
RF receivers are used in many different aspects of wireless communications, from cellular mobile communications to short-ranged WiFi systems. Another RF system with receivers that would use AGC is are television (TV) white space devices. The Federal Communications Committee (FCC) is an independent agency of the United States government that is charged with regulating all non-federal government use of the radio spectrum (including radio and TV broadcasting), and all interstate telecommunications (wire, satellite and cable) as well as all international communications that originate or terminate in the United States. In 2008, the FCC issued rules approving the unlicensed signal operation in the unused TV channels (i.e., white space). However, this approved, unlicensed use is subject to protections set in place for the primary users of the TV band. The primary users of the TV band are Advanced Television System Committee (ATSC)/National Television System Committee (NTSC) transmitters, such as TV broadcasters, and licensed wireless microphones. The new rules allow wireless technologies to use the TV white space as long as the technology and any resulting signal transmissions do not interfere with the existing primary users. For purposes of this disclosure, the various devices that utilize such technologies to access this TV white space will be referred to as “white space devices,” “unlicensed devices,” or the like.
White space devices generally operate in a cognitive manner in which the devices first scan to detect TV band signals from licensed primary users. The white space devices will then select unused channels in order to avoid interference with the licensed signals. Therefore, these white space devices generally share two common functions: (1) sensing for ATSC/NTSC signals; and (2) selecting appropriate channels for interference avoidance. These two functions have different sets of requirements. For example, in performing signal sensing, the FCC dictates that the white space devices should be capable of detecting the non-bursty licensed signals at levels as low as −114 dBm. TV band signals can actually be very strong—at levels as high as −30 dBm. In contrast, for the channel selection functionality, the device will select channels having the minimum interference levels in the presence of bursty interference. Additionally, these white space devices may not consider selecting channels having a received signal strength indication (RSSI) level that exceeds some designated interference threshold. Thus, having such differing functional requirements, design of white space devices can be demanding.